Pioneering French meteorologist Léon Philippe Teisserenc de Bort (1855–1913) conducted an experiment using helium balloons and temperature sensors. He learned that, after about seven miles (11 kilometers), the air stopped becoming cooler and leveled off for as high as the balloons could go. He concluded that the atmosphere was divided into two layers, which he named the troposphere and the stratosphere. Later, in the 1920s, meteorologists Gordon Miller Bourne Dobson (1889–1976) and F.A. Lindemann, First Viscount Cherwell (1886–1957), used studies of meteor trails to learn that temperatures warmed in the atmosphere as high as 30 miles (48 kilometers) up. Dobson concluded that ultraviolet radiation absorbed by the ozone in the stratosphere was the reason for the warmer air.